The present invention relates to a solid electrolytic capacitor using a conductive high polymer in the electrolyte used in various electronic appliances, and a method of manufacturing the same.
Owing to the advancement in digital appliances, recently, capacitors having a low impedance and an excellent high frequency characteristic even in a high frequency region are strongly demanded. To meet such market needs, capacitors using conductive high polymers obtained by polymerizing pyrrole, thiophene or aniline as the electrolyte are being developed and commercially produced.
Hitherto, a solid electrolytic capacitor of this kind comprises, as disclosed in Japanese Laid-open Patent No. 63-158829, an anode electrode made of a valve metal having a dielectric oxide layer, the dielectric oxide layer formed on this anode electrode, a manganese dioxide layer formed by pyrolysis, a conductive high polymer layer on this dielectric oxide layer, and a cathode electrode placed on this conductive high polymer layer. The conductive high polymer layer is formed by electrolytic process in an electrolytic polymerization solution using manganese dioxide layer as the anode.
Formation of conductive high polymer layer by this electrolytic polymerization is quick in forming a conductive high polymer layer as compared with chemical polymerization or vapor phase polymerization, and it requires a relatively simple equipment, and hence it is said to be beneficial for industrial production.
In the prior art, however, the forming condition of manganese dioxide layer has great effects on the principal characteristics of the solid electrolytic capacitor such as capacitance, tan xcex4 and impedance.
That is, when the anode electrode is formed by a method of bonding particles of valve metal into a porous substance by sintering, or a method of multiple etching pits by etching process, such anode electrode has an undulated surface of an expanded surface area of fine pores or the like. The dielectric oxide layer formed on the surface of the anode electrode having fine pores has multiple fine pores and exposed portions reaching the inner depth. In the prior art, the manganese dioxide layer formed on this dielectric oxide layer is formed only on the exposed portions, and not formed in the inner parts of the fine pores. The conductive high polymer layer is formed only on this manganese dioxide layer. That is, in such conventional solid electrolytic capacitor, it was possible to have a cavity in the inside. In such conventional constitution, when the anode electrode has an expanded undulated surface, sufficient capacitance and sufficient impedance corresponding to the expanded undulated surface could not be obtained. Thus, there were serious problems also in the constitution of using conductive high polymer as the electrolyte.
It is hence an object of the invention to present a solid electrolytic capacitor exhibiting a desired effect sufficiently, in the solid electrolytic capacitor using an anode electrode having an undulated surface and a conductive high polymer as electrolyte, and a method of manufacturing the same.
The solid electrolytic capacitor of the invention comprises:
(a) an anode electrode having a first undulated surface,
(b) a dielectric oxide layer placed on a first undulated surface of the anode electrode, in which the dielectric oxide layer includes a second undulated surface placed continuously coinciding with the shape of the first undulated surface,
(c) a manganese dioxide layer placed on the second undulated surface of the dielectric oxide layer, in which the manganese dioxide layer includes a continuous third undulated surface, placed coinciding with the shape of the second undulated surface, on the second undulated surface of the dielectric oxide layer,
(d) a conductive high polymer layer placed on the third undulated surface of the manganese dioxide layer, in which the conductive high polymer layer is placed on the third undulated surface of the manganese dioxide layer, and
(e) a cathode layer placed above the conductive high polymer layer.
The manufacturing method of solid electrolytic capacitor of the invention comprises:
(a) a step of supplying an anode electrode having a first undulated surface,
(b) a step of forming a dielectric oxide layer on the first undulated surface, in which the dielectric oxide layer includes a second undulated surface coinciding with the first undulated surface,
(c) a step of forming a manganese dioxide layer on the second undulated surface, in which the manganese dioxide layer includes a continuous third undulated surface, placed coinciding with the shape of the second undulated surface, on the second undulated surface,
(d) a step of forming a conductive high polymer layer on the third undulated surface, in which the conductive high polymer layer is formed on the third undulated surface of the manganese dioxide layer, and
(e) a step of forming a cathode layer above the conductive high polymer layer.
Preferably, the manganese dioxide layer is placed in contact with the entire surface of concave and convex portions of the second undulated surface.
Preferably, the conductive high polymer layer is placed in contact with the entire surface of concave and convex portions of the third undulated surface.
Preferably, the first undulated surface has a surface with a plurality of fine pores and exposed portions.
Preferably, the anode electrode having the first undulated surface has a porous sinter of valve metal or a roughened metal foil.
Preferably, the conductive high polymer layer has a conductive high polymer layer formed by electrolytic polymerization.
Preferably, it also includes a step of impregnating the manganese dioxide layer sufficiently in a 6.5 wt. % to 26.5 wt. % aqueous solution of manganese nitrate at 10xc2x0 C. to 40xc2x0 C. sufficiently, and lifting, and a subsequent step of removing the excess portion of aqueous solution of manganese nitrate adhered to the surface and a step of heating to more than 80% of pyrolysis temperature within a minute and performing pyrolysis for three minutes or more at 300xc2x110xc2x0 C.
In this constitution and manufacturing method, a manganese dioxide layer can be formed on the entire surface of the undulated surface of the oxide film without damaging the dielectric oxide film of the anode electrode having fine pores or undulated surface. Accordingly, the conductive high polymer layer by electrolytic polymerization is formed securely from the inner surface of fine pores to the outer surface. As a result, a capacitor having the capacitance, impedance, leak current and other characteristics exactly as designed is obtained.